This study is to compare three combination drug therapies in reducing the amount of HIV in the blood, and preventing HIV resistance, and to evaluate the safety of these therapies when given to HIV-infected children who have failed or who are unable to take other therapies. The three combination therapies to be studied are didanosine (DDL)+zidovudine(ZDV or AZT)+evirapine (NVP);DDL+ZDV;and DDL+NVP.